Solids-fluid contacting equipment is, of course, well known. One of the problems associated with such equipment is the entrainment of solids with the treating fluid and the carrying away of such solids with the treating fluid. Such solids often cause problems, and considerable effort is often taken to deal with them, such as by filtering the solids from the fluid. Filtration is perfectly feasible in some systems. However, in others, filtration is difficult and expensive.
A particular problem has arisen in this respect in the decaffeination of green coffee beans. In a well known commercial process, a batch of green coffee beans is introduced into a vessel, and supercritical carbon dioxide is passed through the vessel to effect decaffeination of the green beans. The supercritical fluid, bearing caffeine and leaving the vessel, is then processed, such as with charcoal, to remove the caffeine. The now caffeine-free supercritical fluid is returned to the vessel. During the entire operation, the Circulating carbon dioxide is maintained under extremely high pressure, keeping the carbon dioxide in a supercritical state. The green coffee beans include a considerable amount of chaff. As the beans are processed, some of the chaff is entrained with the supercritical fluid and passes with the supercritical fluid into the supercritical carbon dioxide loop. The amount of entrained chaff is reduced by flowing the carbon dioxide downwardly through the bed. Chaff which does become entrained becomes separated as the carbon dioxide passes through the charcoal bed. As a preliminary step the raw beans may be subjected to chaff removal. Despite such preliminary chaff removal, however, a considerable amount of chaff enters the system with the beans and a considerable amount of chaff will be entrained in the circulating carbon dioxide. After each batch is completed, chaff is removed with the charcoal in which it has accumulated.
A continuous operation for the decaffeination of green coffee beans utilizing supercritical carbon dioxide has been developed and is disclosed in U.S. Pat. No. 4,820,537 issued to Saul N. Katz, the disclosure of which is incorporated herein by reference. In this process, moistened green coffee beans are moved periodically downwardly, in pulses, through a substantially vertical column, while continuously flowing supercritical carbon dioxide upwardly through the column. Unlike the batch process, the flow of supercritical carbon dioxide in the continuous process is not periodically interrupted, and unlike the batch process, the supercritical fluid circulates in its high pressure loop at substantially constant temperature and pressure. Moreover, the system is not intended to be shut down often, and both shutting down and starting up the system require a considerable amount of time. It will be understood that chaff which enters the system with the moistened beans and which becomes entrained in the supercritical dioxide will continuously build up in the system. This build-up can cause serious problems such as the need to shut down the process. Clearly, steps must be taken to deal with any chaff which flows out of the column with the supercritical fluid.
Filters, of course, could be used. However, it has been found that such filtration is extremely inefficient and expensive to put into practice. It has been found, for example, that a filter becomes highly plugged with chaff in a very short time.
It is an object of the present invention to provide, in a method and apparatus for contacting a particulate solid with a fluid in a treatment vessel, means for removing, in an efficient manner, chaff or other solids which becomes entrained in the treatment fluid as it passes through the treatment vessel.